A heat induced crimp is produced in fibers, which may be staple fibers or filaments, that have a differential birefringence and are heated sufficiently. The term crimp refers to the waviness of a fiber and is sometimes measured as crimps per unit of length. Birefringence is a measure of the degree of molecular orientation in a filament. If the molecular orientation across the filament is not uniform, then the fiber is said to exhibit differential birefringence.
Upon heating the fiber, the different molecular orientations in the fiber will exhibit different shrinkage behaviors that result in a random, three dimensional crimp in the fiber. Fibers of this type sometimes are said to have a heat induced or latent crimp because the crimp is induced upon application of heat to the fiber and the degree of crimp depends on the temperature to which the fiber is subjected. Heat induced crimp is to be distinguished from mechanical crimp, which is a machine crimping that produces a regular, ordered crimp, typically in a single plane.
The differential birefringence that produces this random crimp can be induced by any number of methods. Bicomponent fibers, in which the fiber is comprised of two different polymers having different molecular orientations, will show different shrinkage behaviors across the fiber diameter. Differential birefringence also can be produced in a fiber formed from a single polymer by asymmetric quenching of the continuous spun filament. Air and water have been used to cool one side of the filament at a rate different from the other side to produce different molecular orientations across the diameter of the filament.
Latent crimp is also referred to variously as conjugate crimp, helical crimp, spiral crimp, and omega crimp. Fibers having a latent crimp have been used for a variety of purposes. U.S. Pat. No. 3,050,821 discloses that polyethylene terephthalate polyester staple fibers having a three dimensional crimp can be used to produce flannels and suiting fabrics having an acceptable degree of loft and cover. U.S. Pat. Nos. 4,794,038 and 4,783,364 relate to a polyester fiberfill having a latent crimp where the fibers are randomly arranged and entangled in the form of fiber balls.
Despite the various uses of fibers having a three dimensional or spiral crimp in various textile products, there appears to have been no recognition that nonwoven fabrics can be made from these fibers having elastomeric and foam-like compressibility and resilience characteristics. Such a fabric could have a variety of uses and applications, including as a replacement for foam cushions, as a cushioning wrap or padding for articles, as a specialty substrate, or other use. Such a fabric could have the advantage over foam products in that no blowing agents are used to achieve compressibility and resilience characteristics. Such a fabric could also be recycled.